It is useful to understand what is meant by the residual power of a nuclear reactor. When a nuclear reactor is shutdown by introducing a source of high counter reactivity into the core, the number of fissions in the core very rapidly is reduced to a negligible number within a few seconds. However, the radioactive fission products produced in the core of the reactor during normal operation continue to produce a significant amount of power which can amount to several percent of the normal power production of the reactor. Regardless of the cause or the manner in which shutdown is effected, it is necessary to remove such residual power by a reliable means in order to prevent excessive heating of the core, which could lead to shutdown.
Numerous devices exist in the prior art for removing the residual power of a nuclear reactor. These devices are generally characterized as having one or more auxiliary loops, in parallel or branched from the main loops which extract heat from the reactor during normal operation. The auxiliary loops are used for the removal of the residual power only when the reactor is shutdown. The heat emitted by the reactor core is consequently extracted into a cold source by two types of circuits, one constituted by main loops for normal operation and the other constituted by auxiliary loops for shutdown operation and removal of the residual power. Such circuitry requires a system to permit the routing of the heat flux to the appropriate heat source. Typically, such routing takes place by closing or opening mechanical valves on the circuits. A typical prior art system is shown in FIG. 1 which is described below.